Polyvinyl alcohol is typically prepared by alkaline alcoholysis, also known as saponification, of polyvinyl acetate. Yellow coloration of the polyvinyl alcohol is a problem associated with its production and has been attributed to molecular weight, the number of aldehyde groups in the polyvinyl alcohol, and to ash content in the polyvinyl alcohol. It is believed that, since the mechanism of termination in polymerization of vinyl acetate is exclusively by disproportionation, aldehyde end-groups are generated upon hydrolysis. Aldol-condensation of the terminal aldehyde of polyvinyl alcohol with monomeric and oligomeric aldehydes and dehydration by thermal drying lead in general to discoloration due to formation of conjugated double bonds with terminal aldehyde groups. The discoloration becomes more intense the lower the molecular weight of the polyvinyl alcohol and is permanent after the polyvinyl alcohol is dried. Discoloration has also been attributed to degradation of the polyvinyl alcohol when it is subject to high temperatures; for example, temperatures of 100.degree. C. or more.
The presence of yellow color in the polyvinyl alcohol can be troublesome in many applications. For example, when polyvinyl alcohol is used as a coating in papermaking processes or as a sizing agent. It is therefore desirable to produce polyvinyl alcohols that are as low in yellow color as possible. It is also desirable to produce polyvinyl alcohols which are thermally stable; i.e., do not become more discolored with the application of heat.
Various methods have been used in attempts to achieve colorless or very low yellow color polyvinyl alcohol and some of them are described below.
USSR 211,091 (1966) (Derwent Abstract, AN 68-38552Q) discloses the use ortho-phosphoric acid to neutralize the alkali after saponification of vinyl acetate copolymers to produce a white powder product.
U.S. Pat. No. 3,156,667 (Martins, 1964) discloses the use of phosphoric acid to neutralize the polyvinyl acetate hydrolyzates after the conversion to polyvinyl alcohol. The amount of phosphoric acid used is chemically equivalent to 100 to 200% of the alkaline base. This method was reported to produce polyvinyl alcohol with negligible color.
U.S. Pat. No. 3,262,905 (Martins, 1966) discloses treatment of polyvinyl alcohol with alkali metal phosphate to prevent coloration of the polyvinyl alcohol.
U.S. Pat. No. 3,697,495 (Bristol, 1972) discloses treatment of polyvinyl acetate with ozone prior to alcoholysis to produce a polyvinyl alcohol with improved color properties.
JP 48038792 (1973) (Derwent Abstract AN 73-62319U) discloses the addition of hydrogen peroxide or organic peroxide to the reaction system when conversion of vinyl acetate reaches 60% and before saponification is effected to give a white polyvinyl alcohol.
U.S. Pat. No. 4,772,663 (Marten et al, 1988) discloses the production of vinyl alcohol copolymers in which a small amount of an acid, such as phosphorous acid, oxalic acid and citric acid, is added to the vinyl acetate stream prior to the first polymerization vessel, as a means of reducing the color of the product.